In U.S. Pat. No. 4,239,338, Borrelli et al. teach an optical information storage medium comprising a glass substrate, a 1000 Angstrom thick silver layer applied to the substrate, an oxide layer deposited over the silver layer and a multilayer additively colored AgCl/PbO film applied over the oxide layer. The film is optically bleachable using visible light to produce a dichroic, birefringent image. The image is read in infrared light, since the film is transparent at infrared wavelengths. The silver layer permits reading and writing in the reflective mode. The film should have a thickness not exceeding about two microns to permit high spot resolution.
In U.S. Pat. No. 4,278,756, Bouldin et al. teach a reflective laser recording and data storage medium formed from a photosensitive silver-halide emulsion. The emulsion is exposed and developed using a negative silver diffusion transfer process to make the film surface shiny compared to data spots which are clear or dark. The shiny surface may be above or below the main body of the emulsion depending on whether the reading light is to be introduced from above or from below through a clear substrate.
In U.S. Pat. No. 4,542,288, Drexler teaches use of a data card having the media of the kind described in U.S. Pat. No. 4,278,756. Certain information, such as servo tracks and data base information, can be prerecorded photolithographically.
In reflective optical data storage media, high optical contrast between data spots and the media background and sharply defined data spots are necessary for resolving data, particularly where data spots are small, i.e. ten microns or less in size. Improved resolution allows for greater data densities, but is sometimes difficult to achieve. For example, optical contrast may be increased in some media by adding a reflective layer, increasing the reflectivity of an already existing reflective layer, or by decreasing the reflectivity of a photosensitive layer, such as by increasing its thickness. Data density may be increased by reducing the size of data spots. Unfortunately, data spots are often blurred in the process of increasing contrast and shrinking spot sizes due to resolving capability of the media and other effects, such as halation. Halation is distortion seen as blurred or diffuse images caused by reflection of image rays off the back of a photosensitive recording layer thereby setting a lower limit on resolvable spot sizes. Antihalation layers added to some media, reduce the reflectivity from the back of the emulsion layer to produce sharp data spots, but also lower optical contrast Some media, such as that described in U.S. Pat. No. 4,239,338, limit the thickness of the recording layer.
An object of the invention is to increase the data density of reflective media applicable to optical data cards without also increasing the effects of halation and resultant blurring of data spots.
Another object is to create a very high data contrast optical media which is less susceptible to data errors from dust, dirt and scratches.